1. Field of the Invention
The present invention relates to hydraulic circuits that operate machinery; and more particularly to controlling the pressure of hydraulic fluid supplied to power actuators of that machinery.
2. Description of the Related Art
Construction machinery, such as backhoes, have working members which are driven by several hydraulic cylinder and piston assemblies. Each cylinder is divided into two internal chambers by the piston and selective application of hydraulic fluid under pressure to either of the chambers moves the piston in a corresponding direction.
Application of hydraulic fluid to the cylinder often is controlled by a spool valve, such as the one described in U.S. Pat. No. 5,579,642. In this type of valve, a manual operator lever is mechanically connected to a control spool which slides within a bore in a valve body. The pump and tank of the system are connected to ports in the valve body and the cylinder chambers are coupled to workports. Movement of the spool into various positions directs pressurized hydraulic fluid from the pump to one of the cylinder chambers and drains hydraulic fluid from the other cylinder chamber to the tank. By varying the size of orifices through which the fluid flows in the spool valve, the rate of flow into the cylinder chambers can be varied, thereby moving the piston at proportionally different speeds.
To facilitate control, pressure compensating systems have been designed to maintain an approximately constant pressure drop across the spool valve. Those previous systems included sense lines which transmitted the hydraulic pressure at the valve workports to a control input of a variable displacement hydraulic pump. Typically, the greatest of the workport pressures for several working members is selected by a chain of shuttle valves as the pressure to apply to the pump control input. The resulting self-adjustment of the pump output as the workport pressure fluctuates provides an approximately constant pressure drop across the valve orifices whose cross-sectional area is controlled by the machine operator. This facilitates control because, with the pressure drop held constant, the speed of movement of each working member is determined only by the cross-sectional area of the corresponding orifice. Hydraulic systems of this type are disclosed in U.S. Pat. Nos. 4,693,272 and 5,579,642.
There is a trend with respect to construction and agricultural equipment away from manually operated hydraulic valves toward electrically controlled solenoid valves. This change in technology facilitates computerized regulation of various machine functions. Electrical control of system simplifies the hydraulic plumbing as the control valves can be located near the cylinder and not in the operator cab. Thus only a common pair of pump and tank lines needs to be run to the hydraulic actuators throughout the machine. However, for pressure compensation, the load sense pressure lines still must be run among the valve assemblies and the pump. Therefore, it is desirable to achieve pressure compensation without extending load sense pressure lines throughout the machinery.